I am just getting into this software. I construct my circuit, and it tells me to choose a ground. I only have experience with "ideal" circuits and circuit elements, and have only seen "ground" in the context of the node-voltage method for solving circuits. My question is: What does this term "choose a ground" mean, and where should I put it in a simple circuit with a voltage source, a resistor and an LED all in series?
Ground is a notional "0V" point : all voltages are referenced to it. In principle it could be applied to any node in such a simple circuit; however the usual convention is to choose the -ve terminal of the voltage source (or any point directly connected to it, i.e. on the same net). Any other choice runs the risk of confusing people!